Such a method is known from European patent application EP-A 0 908 137. In this application, the strain (deformation) of vessel walls is derived with ultrasound from the relative displacement of a more inward layer and a more outward layer of the vessel wall as a result of the pressure varying through the heartbeat. These relative displacements are (at an assumed equal speed of sound in the medium) equal to the difference of relative time delays of the ultrasound beam, measured at two times.
The relative time delay can be measured by correlating with each other sound signals obtained consecutively over time from one specific direction and deriving the relevant time delay from a correlation optimum. At this optimum, therefore, two signals consecutive over time are maximally correlated when the time difference between the respective signals is equal to the relevant time delay. By taking the difference of time delays measured at two different times and relating this to the time difference between the measuring times, it is possible to derive the degree of strain of the vessel wall in the direction of the sound beam as a result of pressure changes induced by the heartbeat.
By measuring the local relative displacements with a measuring beam in a specific direction and performing this measurement in a measuring plane oriented transversely to the vessel wall, it is possible to display elasticity information about respective measuring positions in the measuring plane. Furthermore, by measuring an average relative displacement along the above directions, a so-called palpogram can be composed, which is indicative of the hardness of the vessel wall in the plane in which the vessel wall cuts the measuring plane. The information derivable from such an elastogram/palpogram is important to identify and characterize plaques on the vessel walls. The composition of plaques can be important to the assessment of their injuriousness to health.
Such information is often not derivable from a conventional echogram, since the image of high-risk cannot be distinguished from less high-risk plaques.
Moreover, practical and theoretical studies show that the degree of strain of the vessel wall is indicative of the stresses that can occur in such plaques. If the stresses become too high, a plaque can tear open, so that a life-threatening thrombosis can arise.
Although for a two-dimensional cross-section, satisfactory measuring results can be obtained, in practice there appears to be a need for a three-dimensional display of the hardness information of the wall, so that the elasticity/hardness of at least one surface part of the vessel wall can be measured. With the present technique, it is practically very difficult to reproducibly analyze a blood vessel in such a manner. Furthermore, on the basis of conventional echographic data it is very hard to localize a suspect spot in a blood vessel. In fact, the performance of a single transverse scan at selected positions in a blood vessel provides insufficient information to enable determination of the presence or absence of plaques in the blood vessel as a whole.